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Last Digit Dim - should I fix or not?
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Last Digit Dim - should I fix or not?

by MichaelaPrahler on Fri Jul 11, 2014 3:55 pm

Hello,
at first i must say that im from germany, so please execuse my bad english!

I have build the tube clock today and all runs fine but the last (second) digit is a lot darker than the other. now I've read a lot in this forum about the problem and have found many fixes. now my question: is there a need to fix this dim digit or is it okay for the lifetime of the tube to let it run like that. the thing is i really love my clock and don't wana mod things i don't understand probertly. so please help.. should I fix it with a resistor at the D3 mosfet or just leave that thing like that?
before I forget.. i have already testet the tube with a jumper at the D3 and the tube diplays all the digits very well and bright.

sorry if I ask double, I have read all the topics with this problem but don't really understand if this is a "must fix" or a "can fix" for my clock.

thanks a lot,
greetings from germany!
MichaelaPrahler
 
Posts: 2
Joined: Fri Jul 11, 2014 3:43 pm

Re: Last Digit Dim - should I fix or not?

by jarchie on Fri Jul 11, 2014 4:44 pm

MichaelaPrahler wrote:at first i must say that im from germany, so please execuse my bad english!

No apology necessary. Your English is excellent!

MichaelaPrahler wrote:now my question: is there a need to fix this dim digit or is it okay for the lifetime of the tube to let it run like that.

I don't think anyone knows with certaintly what the effect of low filament current (a dim digit) will be in the long term. But tubes with a dim digit have worked just fine for many years. My best guess is that tube life will be shortened slightly due to accelerated cathode poisoning--but only by a small amount. So if the dim digit doesn't bother you at all, it should be okay to leave the clock as is.

MichaelaPrahler wrote:the thing is i really love my clock and don't wana mod things i don't understand probertly. so please help..

The problem is fairly straightforward: Q3 acts like a switch to provide power to VFD driver chip and filament. For some tubes, the Q3 included in the Adafruit kit is not powerful enough to provide sufficient current for the IV-18 filament. Without sufficient current, the filament does not get hot enough at the ends to fully light neighboring digits.

The solution is also fairly straightforward: Replace Q3 with a better switch--a switch that can provide more power. There are two common fixes.

(1) Remove Q3 and install a P-MOSFET that can provide more power, such as the ZVP2110A, available from Digi-Key (ZVP2110A-ND) or Mouser (522-ZVP2110A).

(2) Remove Q3 and install a bipolar PNP transistor with a resistor at the base, such as a PN2907A and 1k, also available from Digi-Key (PN2907ATACT-ND and 1.0KQBK-ND) or Mouser (512-PN2907ABU and 291-1K-RC). There's a good picture here.

Both solutions have been tried and have worked for many users on this forum.

MichaelaPrahler wrote:should I fix it with a resistor at the D3 mosfet or just leave that thing like that?

It should be fine to leave the clock as is. But the solution is fairly simple, so why settle for an uneven display?

jarchie
 
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Location: Santa Cruz, California, United States

Re: Last Digit Dim - should I fix or not?

by MichaelaPrahler on Fri Jul 11, 2014 5:22 pm

Wow! Thank you so much for your answer(s) and the technical background!!

It should be fine to leave the clock as is. But the solution is fairly simple, so why settle for an uneven display?

You're right! If it is that simple and safe I think it will be worth changing the D3 to have a lovely bright display. :) I'm going to buy a ZVP2110A and replace the original mosfet with this one, also going to study more about the technical background of this clock and it's tube (I'm a absolutly beginner in this stuff).

Have a nice Evening!
MichaelaPrahler
 
Posts: 2
Joined: Fri Jul 11, 2014 3:43 pm

Re: Last Digit Dim - should I fix or not?

by jarchie on Fri Jul 11, 2014 6:45 pm

MichaelaPrahler wrote:Wow! Thank you so much for your answer(s) and the technical background!!

You're very welcome! Glad it was helpful.

MichaelaPrahler wrote:If it is that simple and safe I think it will be worth changing the D3 to have a lovely bright display. :) I'm going to buy a ZVP2110A and replace the original mosfet with this one...

Yes, it is simple and safe. The ZVP2110A is a pin-compatible replacement for the ZVP3306A that came with the kit. So after removing the ZVP3306A, the ZVP2110A should be installed the same way:

ZVP2110A.jpg
The ZVP2110A that is installed in my Adafruit clock.
ZVP2110A.jpg (39.3 KiB) Viewed 467 times

MichaelaPrahler wrote:...also going to study more about the technical background of this clock and it's tube (I'm a absolutly beginner in this stuff).

I highly recommend the Noritake VFD guide to learn about VFDs. It's an excellent compliment to the Adafruit design notes.

jarchie
 
Posts: 595
Joined: Sun Jun 24, 2012 2:16 pm
Location: Santa Cruz, California, United States

Please be positive and constructive with your questions and comments.